For releasing an active substance into the surrounding atmosphere under a controlled condition, various kinds of slow releasing devices are known For example, there are devices encapsulating an active substance in microcapsules as described in U.S. Pat. Nos. 4,286,020 and 4,353,962 and devices enclosing an active substance in tubes or capillaries as described in JP-A-52-55969 and 57-45101 (the term "JP-A" as used herein means an "unexamined published Japanese patent application), and U.S. Pat. No. 4,017,030. Furthermore, there is also proposed a device of impregnating a porous regin molding having continuous perforations with an active substance as described in JP-A-59-13701. U.S. Pat. No. 3,784,488 discloses a film forming solution composition. The composition is prepared by dispersing an excess of oil components into a soft polymer structure, and is applied to a skin of human body to form a film, thereby maintaining the effect of a sunscreen oil. U.S. Pat. No. 4,138,344 discloses a device wherein a solid active substance is despersed in a polymer matrix which is decomposable in a living body, and as the polymer dissolves, the active substance gradually releases.
However, in such active substance slow releasing devices, an active substance is generally released for a long period of time and after passing a period capable of effectively releasing the active substance, the releasing rate of the active substance is reduced and finally the activity thereof is vanished but it has hitherto been difficult to clearly confirm the diminished effect of the active substance.
Furthermore, since the releasing rate of an active substance from such a slow releasing device greatly deviates according to the surrounding atmospheric conditions such as temperature, humidity, etc., and the period of effectively releasing the active substance, that is, the life of the active substance is not always constant, the effective period indicated on the device is a simple prospective criterion and is not a highly reliable term. Accordingly, conventional slow releasing devices are frequently still used even vanishing the activity of the active substance or they are frequently discarded as activity vanished devices in spite of that they still retain the activity.
For solving the aforesaid problems in conventional active substance slow releasing device, a device wherein a solid active substance is dispersed as fine solid particles in a substantially transparent and inactive resin in an amount of more than the saturated solubility is proposed as described in JP-B-U-57-31067 (the term "JP-B-U" as used herein means an "examined published Japanese untility model application"). According to the device, since the device is opaque when a sufficient amount of the active substance exists in the molding in a sufficient amount but the resin molding becomes substantially transparent when the greater part of the active substance is evaporated or dissolved off from the molding and the existing amount thereof in the resin molding reaches the saturated solubility, the vanishment of the acitivity of the device can be visually confirmed.
On the other hand, as a device for slow releasing an active material which is liquid at normal temperature, there is proposed a device of enclosing a liquid active material having a limited solubility for a polymer in a layer of the polymer in an amount of the solubility as independent fine droplets as described in JP-A-59-59733, However, in such a device, since the polyer molding or layer retains the independent fine pores and has the porous structure after releasing the liquid active substance different from the aforesaid case that the active substance is solid, the device does not become transparent and is opaque, thereby the vanishment of the active substance can not visually confirm.